You can use the Subnets page to add a description for the Subnet resource for the target platform.
In the Web Interface toolbar, select the Platforms tab.
Select the target platform, then select the Networks tab (or Virtual Private Networks tab) under Associations, then select Subnets.
In the Subnets list, select the resource, then click Edit (or double-click the network) to open the Edit Subnet dialog.
Modify the Description for the resource.
Click Save to update the Subnet resource.
Click Close to exit the dialog.